Since the first experimental use in 1947, hydraulic fracturing, commonly known as fracking, has been gradually adopted for the stimulating treatment of oil wells and has become a great success in the past twenty years, especially in North America. High pressure pumping systems to propel the fracturing fluid into the wellbore is critical to successful fracking operations. The key component of such systems is a high pressure reciprocating plunger pump, comprising a power end and fluid end, which has been widely used in oilfield applications for several decades. The power end converts the rotation of a drive shaft to reciprocating motion of a plurality of plungers. The reciprocation motion of the plungers, in association with the operation of valves within the fluid end, produces a pumping process due to the volume evolution within the fluid end. Typically, the fluid end is comprised of a pump housing, valves and valve seats, plungers, seal packings, springs and retainers. The pump housing has a suction valve in the suction bore, a discharge valve in the discharge bore, an access bore and a plunger in the plunger bore. In the suction stroke, the plunger retracts along the bore and causes a quick decrease of the inner pressure; thus, the suction valve is opened and the fluid is pumped in due to the pressure difference between the suction pipe and the inner chamber. In the forward stroke, the hydraulic pressure gradually increases until it is large enough to open the discharge valve and thus pump the compressed liquid into the discharge pipe.
The pump housing is cyclically strained during the reciprocating motion of plungers. The cyclic hydraulic pressure causes the initiation of fatigue crack in the intersecting bores of the pump housing made of high-strength forged steels. Severe wear can also be observed in the cross-bores of fluid end after the operation, causing the leaking or emission of the fluid.
Additionally, the fracking fluid injected into the wellbore at high pressure generally contains fracture sand, chemicals, mud and/or cement. These chemicals are used to accelerate the formation of cracks in reservoirs and the small grains of sands hold formed cracks open when hydraulic pressure is removed, but these additives also accelerate the damage of the components of the high pressure pumping system, which are already under heavy duties, and bring challenges to the pump manufactures.
Nowadays, hydraulic fracturing has changed along with the rapid exploitation of shale gas in more complex geological formations to ensure energy supply worldwide. The evolution of high pressure pumps has occurred throughout the development of hydraulic fracturing with the increase of both pressure capabilities and flow rate. Conventional fracturing operations in gas wells require only one or two fracturing stages to complete the stimulation process of a vertical well, and the required pressure is most often less than 10,000 psi; thus, the pump using a simple design is capable of meeting the demands. However, the pumping environment becomes harsher when the unconventional resources (e.g., Barnett Shale and Haynesville Shale) are commercially developed with horizontal drilling techniques in the past decade. The stimulation process requires higher pumping pressure (up to 13,500 psi) and much longer pumping time (nearly all hours of every day), causing accelerated stress damages and increased wear of expendable components, including the fluid ends. Therefore, pump manufacturers are now exploring modifying existing pump models to improve the duty cycle and extend operating life in these harsher environments.
In order to enhance the durability of high pressure pumps, the engineers and researchers need to battle with the fatigue of metals through optimization of the structure and materials. Fatigue is a progressive and localized structural damage process that occurs when a material is subjected to cyclic loading. It is dangerous and unwanted because components could fail under much lower stress than the fracture strength. Fatigue failure processes depend on the cyclic stress state, geometry, surface integrity, residual stress and environment (temperature, air or vacuum or solution), etc. The relationship between fatigue life and the applied stress can be approximately represented by the Basquin Equation:Sa=A×(Nf)B Where Sa is the effective alternating stress, Nf is the corresponding cycle number when failure occurs, and A and B are the fitted parameters (A>0 and B<0). When the applied stress Sa increases, the corresponding lasting cycles Nf would decrease. Thus, the higher stress requirements for stimulating shale gas reservoirs accelerate the fatigue damages of pumping systems. In addition, the concept of stress concentration (k), an amplifying factor for applied stress due to geometry effect, is basically related to the likelihood of fatigue and/or stress corrosion cracking of pump housing. The working pressure (P, less than 20,000 psi) in oilfield is much smaller than the endurance limit of high strength steels (e.g., 100,000 psi for 4330 steel); but the effective stress Sa (=k×P) is pretty close to the fatigue limit of steels when the factor k is larger than 5 due to the intersecting geometry of fluid end.
The breakdown of high pressure pumping system can cause significant problems in the oilfield. The downtime for replacement or maintenance of fluid ends at the fracturing site costs the oil service companies tens of thousands of dollars; plus, the users need to have significant excess backup of pumping equipment to ensure continuous operation, which is counter to the current emphasis on shrinking the oilfield footprint. Therefore, the best solution is that pumping products with greater reliability and predictability be provided through technology innovations to meet the challenging requirements. Prior art techniques have included using hand grinding radii at the intersection of the fluid end bores or using obtuse intersecting angle design (e.g., Y-type pump) to reduce the stress concentration. In addition, because the fatigue failure at intersecting bores is initiated from the surface under tension stress, a strategy to counter such failure mechanism is to pre-stress the surface in compression, including “shot peening” at the intersecting port, autofrettage treatment of the whole fluid chamber or using a tension member longitudinally extending through the pump body to apply compressive stress. But none of these prior art techniques have satisfactorily addressed the difficulties. The shot peening-induced compressive layer is too thin to protect the inner surface from “sand erosion.” The hydraulic pressure required for the effective “autofrettage” treatment is high (close to 70,000 psi) and has the potential to cause damage inside the chamber.
The present invention relates to reducing the effective stress applied on fluid ends of high pressure plunger pumps through structural changes to thus mitigate or eliminate the fatigue and stress corrosion cracking of high pressure components.